For a power amplifier for transmission used in radio communication equipment such as a portable telephone or wireless LAN, such a scheme that a trap circuit short-circuiting impedance with respect to a frequency twice as high as a frequency used for transmission of a signal (hereinafter, referred to as a transmission frequency) is provided within an output matching circuit has widely been employed.
By providing a trap circuit within the output matching circuit, radiation from the power amplifier, of second harmonic resulting from a non-linear operation of a power amplification element constituting the power amplifier can be suppressed. In addition, by arranging the trap circuit such that impedance of the output matching circuit with respect to the frequency twice as high as the transmission frequency on the side of the power amplification element is short-circuited, the power amplifier can operate with high efficiency.
FIG. 7 illustrates an exemplary conventional power amplifier (hereinafter, referred to as the first conventional example). Here, a simplified version of the figure in Japanese Patent Laying-Open No. 07-022872 is shown, illustrating a configuration of main portions in the power amplifier where a trap circuit implemented by an open stub short-circuits impedance with respect to a frequency twice as high as a transmission frequency.
Referring to FIG. 7, the conventional power amplifier includes an input terminal 101 of the power amplifier, an input matching circuit 102, a power amplification element 103, an input-side bias circuit 104 of the power amplification element, an output-side power supply circuit 105 of the power amplification element, a bias supply terminal 106 of input-side bias circuit 104, a power supply terminal 107 of output-side power supply circuit 105, an output matching circuit 108, and an output terminal 109 of the power amplifier.
Input matching circuit 102 receives a signal from input terminal 101 and outputs the signal to the base of power amplification element 103 implemented by a GaAs hetero-junction bipolar transistor. In addition, input-side bias circuit 104 is provided between bias supply terminal 106 and the base of power amplification element 103. Power amplification element 103 has the emitter electrically coupled to a ground GND and the base coupled to an output node of input matching circuit 102 and input-side bias circuit 104. Output-side power supply circuit 105 is provided between the collector of power amplification element 103 and power supply terminal 107.
Output matching circuit 108 is constituted of a microstrip line 110 and a matching circuit 111, and an open stub 112 serving as the trap circuit is provided at a connection point of the collector of power amplification element 103 and microstrip line 110.
Open stub 112 has a length of λg2/4 (λg2 represents a propagation wavelength of second harmonic), and serves as the trap circuit for short-circuiting impedance of output matching circuit 108 with respect to the frequency twice as high as the transmission frequency on the side of an output end of power amplification element 103. Matching circuit 11 is provided between microstrip line 110 and output terminal 109.
The second harmonic generated from power amplification element 103 is reflected under the influence of open stub 112 serving as the trap circuit, and consequently, the power amplifier operates with high efficiency and the second harmonic output from output terminal 109 of the power amplifier is suppressed.
FIG. 8 illustrates another exemplary conventional power amplifier (hereinafter, referred to as the second conventional example). Here, a simplified version of the figure in Japanese Patent Laying-Open No. 09-046148 is shown, illustrating a configuration of main portions in the power amplifier where the trap circuit in which a series resonant circuit including an inductor L and a capacitor C is connected to ground GND short-circuits impedance with respect to the frequency twice as high as the transmission frequency.
Referring to FIG. 8, another conventional power amplifier is different from the power amplifier in FIG. 7 in that output matching circuit 108 is replaced with an output matching circuit 108P.
Output matching circuit 108P includes capacitors 113 to 117, inductors 118 and 119, and a microstrip line 120.
A series resonant circuit consisting of capacitor 115 and inductor 118 is provided between ground GND and microstrip line 120, thus serving as a trap circuit 121. A series resonant circuit consisting of capacitor 116 and inductor 119 is provided between ground GND and microstrip line 120, thus serving as a trap circuit 122. A resonant frequency of the series resonant circuits is twice as high as the transmission frequency, and trap circuits 121 and 122 serve as the trap circuits for the twice higher frequency.
Here, two trap circuits are provided, whereby an effect thereof is enhanced. Microstrip line 120 has a length of λg/4 (λg represents a propagation wavelength of the transmission frequency), and short-circuits impedance of the output matching circuit, with respect to the frequency twice as high as the transmission frequency on the side of the output end of power amplification element 103. The second harmonic generated from the power amplification element is reflected under the influence of these series resonant circuits serving as the trap circuits, and consequently, the power amplifier operates with high efficiency and the second harmonic output from output terminal 109 of the power amplifier is suppressed.
Meanwhile, in radio communication systems in recent years, depending on countries or regions, frequencies allocated to the systems slightly vary even though the same communication scheme is employed. For example, in the case of a wireless LAN system in a 5 GHz band, a frequency around 5.2 GHz is used in Japan and the like, while a frequency around 5.8 GHz is used in North America and the like. In order to address such a situation, in the power amplifier for transmission used in the recent radio communication systems, a single power amplifier is required to operate over a wide frequency band.
The conventional techniques, however, do not address suppression of the second harmonic over a wide frequency band and short-circuiting of impedance of the output matching circuit with respect to the frequency twice as high as the transmission frequency on the side of the power amplification element.